Our studies of the mutual adhesion of dissociated chick embryonic retinal cells have demonstrated the presence of two different adhesion systems. One is Ca ions-independent and nearly temperature-independent, whereas the other is Ca ions-dependent and strongly temperature-dependent. Antibodies raised against the two different systems and rendered univalent inhibit only the corresponding adhesion system and do not "cross-inhibit". We are now working to clarify the roles of these two adhesion systems and to identify the corresponding cell surface receptors. Microsurgical manipulations of the amphibian pronephric duct have provided us with evidence that in urodeles and certain anurans the duct is guided along its path to the cloaca by adhesive differentials. We are now evaluating indications that the duct's path is only one track on an "adhesive landscape" encompassing the entire embryonic mesoderm mantle and serving to guide the movements of primordial germ cells as well as of the pronephric duct. Finally, we are using chemical and immunological tools to investigate the role of fibronectins in intercellular adhesion and morphogenesis in several selected multicellular systems.